Bitumen is a material that is commonly used for the preparation of paving and roofing materials and for coatings such as for pipes and tank liners. In the road construction and road paving industry, it is a well-practised procedure to coat aggregate material such as sand, gravel, crushed stone or mixtures thereof with hot fluid bitumen, spread the coated material as a uniform layer on a road bed or previously built road while it is still hot, and compact the uniform layer by rolling with heavy rollers to form a smooth surfaced road.
The combination of bitumen with aggregate material, such as sand, gravel, crushed stone or mixtures thereof, is referred to as “asphalt”. Bitumen, also referred to as “asphalt binder”, is usually a liquid binder comprising asphaltenes, resins and oils. It may be naturally occurring, but may also be obtained from the residues of crude oils, e.g., by fractionation or by precipitation, e.g., by means of propane, or obtained after refining processes of crude oils, such as cracking. Bitumen usually contains hydrocarbons with a high asphaltene content, e.g., 12% wt or more. The bitumen may also have undergone some further treatment, e.g. blowing, whereby bitumen components are subjected to oxidation with oxygen, e.g. air, or a chemical component, e.g. phosphoric acid.
It is known to modify the properties of bitumen by the addition of polymers, in particular, thermoplastic rubbers. This has been described in U.S. Pat. No. 5,718,752. According to this patent document a bitumen-polymer mixture should meet a number of requirements in order to be suitable for paving materials. One of these requirements is that components of the mixture must be compatible. The polar asphaltene fraction tends to be incompatible with the polymer, which may lead to phase separation and therefore to a serious deterioration of the physical properties. Another requirement is the right flow behaviour to prevent rutting. It is therefore important that the mixture shows a satisfactory elasticity, since the material must be able to recover rather than just resist deformation. This characteristic is very important in a warm climate.
The bituminous compositions provided in U.S. Pat. No. 5,718,752 exhibits improved processability and compatibility, and comprise a specific radial block copolymer of polymer blocks of a vinylaromatic compound, such as styrene, and polymer blocks of a conjugated diene, such as butadiene.
In U.S. Pat. No. 5,863,971 it has been disclosed that high temperature rutting resistance and improved ageing behaviour can be accomplished if a hard blown bitumen is mixed with a thermoplastic rubber. Suitable thermoplastic rubbers include optionally hydrogenated block copolymers of a monovinyl aromatic compound, such as styrene, and a conjugated diene, such as butadiene or isoprene. The properties of the compositions with a hard bitumen are shown to be better than those of compositions with a softer bitumen that has not been subjected to a blowing treatment.
Compatibility problems between bitumen and polymers have been addressed in GB-A 2384240. Thereto, bitumen and the polymer were mixed, the mixture obtained was subjected to a blowing treatment and sulphur was added to the mixture during the blowing treatment. The purpose of the sulphur addition was to obtain cross-linking between the polymer molecules. Sulphur was added in an amount ranging from 0.01 to 2% wt, based on the weight of the bitumen and the polymer. The process resulted in products that were homogeneous.
The addition of sulphur to polymer-modified bitumens comprising styrene-butadiene or styrene-butadiene-styrene is addressed in detail by Martinez-Estrada et al in the Journal of Applied Polymer Science, Vo. 115, 3409-3422 (2010). The addition of relatively small amounts of sulphur is shown to greatly increase the thermal stability of the modified bitumens. However, it is stated that precise dosing of sulphur is extremely important as slight excess leads to gel formation. The skilled person is aware that addition of small amounts of sulphur may be beneficial, but would be wary of adding larger amounts due to the risk of gellation.
Bitumen compositions that contain sulphur and polymer have been described in WO-A 03/014231. These known compositions have been developed to provide improved paving binders. To obtain improved paving binders sulphur is added to a bitumen binder and aggregate, sand or other materials. The sulphur acts as a so-called asphalt additive and is used to render the binder less flowable. According to WO-A 03/014231 the paving binder may comprise polymers or polymerisable materials as further constituents. Examples of polymerisable material or polymers are styrene monomer, polyethylene terephthalate, ethyl vinyl acetate, Exxon 101 or Exxon 103 and other vinyl aromatics. The specification is silent about the effect these polymerisable compounds may have on the paving binder or the eventual asphalt.
It has now surprisingly been found that the elasticity of asphalt compositions containing polymer-modified bitumen can be further enhanced by the addition of sulphur. The inventors have added significant quantities of sulphur (from 20 wt %) to bitumen compositions, without experiencing the gellation that has been mentioned in the prior art. These bitumen compositions have advantageously been used to provide asphalt compositions with enhanced elasticity.